Spend some time in Williams’s lab and you start to understand why a lot about molecular electronics is still a mystery, beginning with the relatively simple question of what exactly the researchers are building. Yong Chen, a native of China and a member of Williams’s group since 1998, spends a lot of his time sitting in a stuffy, windowless, nine-square-meter room padded with thick foam. It’s the home of a delicate electron microscope, which uses electron beams to create a rough picture of the structures Chen creates in the laboratory down the hall.Chen is the leader of the team that has given the group its biggest public success to date, the 64-bit crossbar memory. His team first imprinted eight parallel nanowires made of titanium and platinum on a silicon substrate, and covered these wires with a one-molecule-thick layer of a synthetic chemical called rotaxane. They then deposited a second set of titanium wires perpendicular to the first, creating the possibility of an electrical connection between the wires at any junction in the grid.
Each molecule of rotaxane-which was invented by chemist Fraser Stoddart at the University of California, Los Angeles-consists of a long axle with two lumps of atoms at each end, and a ring of atoms circling the axle. Stoddart and Williams’s groups theorize that when a voltage is applied through a specific, intersecting pair of nanowires, the rings on the rotaxane molecules between the wires “jump” from one end of the axle to the other and stay there until another voltage is applied. This could raise or lower the molecules’ resistance to electrical current, and these two states of conductivity would represent digital 1s or 0s. Now Chen, eager to see how small he can make such a device, is trying to print the individual wires even closer together. It’s painstaking labor, where you never know if you’re making progress until the moment it works.
Today Chen is open mouthed, rapt, focusing absolute attention on the monitor in front of him, while also trying to carry on a conversation. He is not entirely successful. Several minutes pass quietly as a question hangs in the air, unanswered. He increases the microscope’s magnification as he searches through a series of fuzzy, gray-on-gray images that look like satellite photos of a desert.
“After we finish the fabrication process, we come in here to check what kind of thing we have got,” he says. “I want to see if the wire is grounded to the substrate or suspended above it. There’s one. Oops, I lost it.”
Eventually he finds something that looks like a length of rebar on a pile of charcoal dust but is actually a wire, 35 nanometers in width, resting on the silicon base. He takes a picture, silent again, holding his breath since sound waves will affect the quality of the image.
“We can talk now,” he says. “Here, in fact, you can see this wire is broken. Too bad. This is a routine experiment, frankly.” Chen’s goal is to find a combination of materials-a “recipe,” if you will-that will impart a Teflon-like non-stickiness to the mold that deposits the wires on the substrate; otherwise they bulge and twist when the mold is removed. But sitting in this hushed, foam-covered room, watching one of the leading scientists in the field searching through grainy images, you realize just how difficult it is to work on this scale. Three weeks later, after five months of painstaking experiment and observations, Chen and Gun-Young Jung find the result they were looking for, bringing the possibility of molecular-sized circuits a small step closer.
“I miscalculated several things,” Chen says simply.
Now he can move on to the next problem.
Observing results, of course, is the last step in a train of events that traditionally begins with a theory about how things should behave. In the case of molecular electronics, though, very little has run a straight course from theory to experiment to result. Theories can languish for years waiting for tools precise enough to test them. In fact, chemists first proposed the idea of molecular electronics in the mid-1970s, but another 20 years would pass before anyone could begin to put it into practice. Lately, though, experimental results have begun to leapfrog the ability of theorists to explain them.
One puzzle is the lack of consistency in measuring experimental results, from lab to lab and even from experiment to experiment. Alex Bratkovsky, a theoretical physicist and native of Moscow who joined HP in 1996, says he was one of the first to realize that a molecule’s orientation between metal electrodes is critical to understanding its switching properties. “The current depends tremendously on how the molecule connects with the substrate,” Bratkovsky says. “The signal may go away, then come back, depending on the position of the molecule. We disregarded that fact for quite a while.” Since controlling the orientation of the molecule is still beyond current experimental tools, results vary widely from lab to lab, and scientists need to judge in many instances whether differences between their results have real meaning or can be explained by effects still outside of experimental control.
To understand the switching phenomenon, the HP researchers are studying a range of new molecules that might be controlled more easily than rotaxane, Bratkovsky says. Some of these are already being designed, but progress is slow. It can take more than two years to design, simulate, synthesize, and finally test a molecule for its electronic properties-after which researchers may find themselves beginning all over again.
Across the hallway from Bratkovsky, Duncan Stewart, an experimental physicist recently hired by Williams’s lab, spent more than six months on a contrarian experiment to help investigate why some molecules can act as molecular switches, changing their conductivity in response to an applied voltage. Instead of designer molecules like rotaxane, Stewart used a simple hydrocarbon molecule consisting of a chain of 18 carbons surrounded by hydrogen atoms. Stewart calls it the “Plain Jane of the molecular world.” It’s stable, inert, and theoretically should have no interesting electronic properties. But it switched anyway.
“I have heaps of data, and the story is that the data do not fit any model, or any existing theory. So even in the simplest case, we don’t understand how electrons are traveling through a molecule,” he says. “At times it’s extremely frustrating. You have to be very pigheaded, beat your head against a wall for six months, and eventually a single brick budges, and eventually the whole wall crumbles and you see another wall.”